save level, save all new spawned objects,.. how to :)

Hi,
i have a plan to save the level, but i have no clue yet how to. I can save all kinds of variables, but i would like to save the level itself. I have a kind of building level, where kids can build something from spawned objects (from class). So the level is after half an hour full of nice buildings. But is there a easy way i can save this all? I thought it was easy to do this, but now i am working on it, its harder then i thought. Anyone an idea how to set this up?